[Crash] Fixed crash after closing first app window

Always create BrowserWindow with MainApp::createWindow (even in MainApp
constructor).
Regression from recent commits.
[CloseOnLastTab] Changed behavior of "Don't quit upon closing last tab" option

It is now "Don't close window upon closing last tab", as it is more
reasonable to bind it to window, not the whole app.

Instead of loading the url-on-new-tab, it now simply blocks closing last
tab